/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.iotdb.lsm.wal;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.*;
import java.nio.file.Files;
import java.util.NoSuchElementException;

/**
 * get records in wal file
 */
public class WALReader implements IWALReader {
    private static final Logger logger = LoggerFactory.getLogger(WALReader.class);
    // wal file
    private final File logFile;
    // wal record prototype, clone on read
    private final IWALRecord prototype;
    private DataInputStream logStream;
    // next wal record
    private IWALRecord nextRecord;
    private boolean fileCorrupted = false;

    public WALReader(File logFile, IWALRecord prototype) throws IOException {
        this.logFile = logFile;
        this.logStream =
                new DataInputStream(new BufferedInputStream(Files.newInputStream(logFile.toPath())));
        this.prototype = prototype;
    }

    @Override
    public void close() throws IOException {
        logStream.close();
        logStream = null;
    }

    @Override
    public boolean hasNext() {
        if (nextRecord != null) {
            return true;
        }
        try {
            if (fileCorrupted) {
                return false;
            }
            int logSize = logStream.readInt();
            if (logSize <= 0) {
                return false;
            }
            // first clone the object through the prototype
            nextRecord = prototype.clone();
            // then perform deserialization and assign a value to the new object
            nextRecord.deserialize(logStream);
        } catch (EOFException e) {
            logger.info(e.getMessage());
            return false;
        } catch (IOException e) {
            logger.warn(e.getMessage());
            fileCorrupted = true;
            return false;
        }
        return true;
    }

    @Override
    public IWALRecord next() {
        if (nextRecord == null) {
            throw new NoSuchElementException();
        }
        IWALRecord walRecord = nextRecord;
        nextRecord = null;
        return walRecord;
    }

    @Override
    public String toString() {
        return "WALReader{" + "logFile=" + logFile + '}';
    }
}
